I just installed Zork 1, 2, and 3 on my Mac running OS X. I used to spend hours hunched over our Apple ][+, trying to work my way through the game, creating hand-drawn maps to help keep from getting lost, etc. Now, I can fire it up anytime I want to relive those glory days of text-based adventure games.

Just curious if you're also a fan of the old game Marathon. If so, you should be aware of the Aleph One project. Bungie open-sourced the game engine code, and recently released the scenario files for free too. The graphics are kind of old-school, but it's still a great game: plot, mystery, puzzles, and lots of firepower. Runs on Mac, Windows, and Linux too.